In electrostatographic imaging and recording processes such as electrophotographic copying, an electrostatic latent image formed on a photoconductive surface is developed with a thermoplastic toner powder, which is thereafter fused to a receiver. The fuser member can be a roll, belt, or any surface having the suitable shape for fixing thermoplastic toner powder to the receiver. The fusing step commonly includes passing the receiver, for example, a sheet of paper on which toner powder is distributed in an image-wise pattern, through the nip of a pair of rolls. At least one of the rolls is heated; in the case where the fuser member is a heated roll, a smooth resilient surface is bonded either directly or indirectly to the core of the roll. Where the fuser member is in the form of a belt, it is preferably a flexible endless belt having a smooth, hardened outer surface that passes around the heated roller. A persistent problem with electrostatographic fusing systems, known as offset, is the adhesion of heat-softened toner particles to the surface of the fuser member rather than the receiver during passage through the rolls. Any toner remaining adhered to the fuser member can cause a false offset image to appear on the next sheet that passes through the rolls and can also degrade the fusing performance of the member. Another possible problem is degradation of the member surface caused by continued heating, which results in an uneven surface and defective patterns in thermally fixed images.
Toner fuser rolls are composed of a cylindrical core that may include a heat source in its interior, and a resilient covering layer formed directly or indirectly on the surface of the core. Roll covering layers are commonly made of fluorocarbon polymers such as TEFLON™ or silicone polymers such as poly(dimethylsiloxane) polymers. A thin layer of a suitable primer may be advantageously coated on the surface of the core to improve its bonding of with the covering layer. For example, U.S. Pat. No. 5,948,491 discloses a priming agent composition that is useful for binding a toner fuser member surface layer containing a fluorocarbon polymeric material to a metallic or an organic polymeric substrate. An intervening layer of the described priming agent composition, which comprises about 98 to 99.99 wt. % of a cross-linked glycidyl end-capped bisphenolic polymer and about 0.01 to 2 wt. % of a perfluoroalkylsubstituted glycidyl-reactive compound, improves the adherence of a surface layer formed from an amorphous fluoroplastic material such as TEFLON™ to the fuser member substrate.
Frequently, release oils composed of, for example, poly(dimethylsiloxanes) are applied to the fuser roll surface to prevent adherence of toner to the roll. Such release oils may interact with the resilient layer upon repeated use and in time cause swelling, softening, and degradation of the roll. Silicone rubber covering layers that are insufficiently resistant to release oils and cleaning solvents are also susceptible to delamination of the roll cover after repeated heating and cooling cycles.
Toner fuser belts are composed of a continuous flexible material having superior resistance to heat and a smooth surface. The belt substrate can be metallic or polymeric. As described above for fuser rolls, the surface layer of the belt is typically a thin coating of a low surface energy polymer such as a fluorocarbon or a silicone resin. This outer layer should adhere strongly to the belt substrate and form a hard, tough surface that is resistant to wear and cracking as well as to cleaning solvents and fluids.
In electrostatographic imaging processes dry developers can be used to form an image on a receiving surface such as a sheet of paper. Dry developers usually include a toner powder and carrier particles. Carrier particles and toner particles have different triboelectric values. As the developer mixture is agitated, the particles rub together and the toner and carrier particles acquire opposite electric charges and cling together. In the subsequent development step the somewhat higher opposite charge of the electrostatic latent image draws the colored toner from the carrier and develops the image. Various addenda are frequently used to improve the properties of the toner and carrier particles.
Toners include, as a major component, the binder and, as minor components, a colorant, and a charge control agent. The binder can be any resin having properties suitable for dry toners. Many such resins are known, but thermoplastic resins that are fixable by fusing are especially useful. When a dry toner powder image is transferred from one surface to another, defects in the image can occur. It is known from U.S. Pat. No. 4,758,491 that the addition of low surface energy liquid or solid addenda, especially polymers containing organopolysiloxane segments, may alleviate many of these defects.
Carrier particles include magnetizable irregular particles that are usually coated with a film of a polymeric material, which helps develop the triboelectric charge and aids the transfer of the toner. The coating material must adhere well to the carrier particle because the toner charge decreases as the polymer wears off. Polymers with low surface energy properties are especially useful for coating carrier particles.
Recent electrophotographic apparatus and processes are disclosed in U.S. Pat. Nos. 5,089,363 and 5,411,779, the disclosures of which are incorporated herein by reference.
U.S. Pat. No. 5,411,779 describes an apparatus having an image-fixing belt with a polyimide resin inner layer and a fluoroplastic outer layer that produces unglossed, matte images. Other fuser belt systems are described in U.S. Pat. Nos. 5,200,284; 5,233,008; 5,330,840; 5,362,833; and 5,529,847, the disclosures of which are incorporated herein by reference.
The ferrotyping belt used for the production of high gloss toner images typically has a metal or an organic polymeric substrate on which is coated a release layer. The toner is generally fused in a heated nip to a receiver, which then continues to travel along the belt without releasing until the toner is cool. To avoid the use of a release oil, the release layer of the fuser belt must have low surface energy.
Toner fuser belts are composed of a continuous smooth, heat-resistant, flexible material on a metallic or polymeric substrate. A release layer applied to the belt substrate is a thinly coated, low surface energy polymer such as a fluorocarbon or a cross-linked silicone resin. Such release layers, however, often display poor mechanical properties, including inadequate adhesion to the metal support, and are susceptible to rapid wear upon repeated contact with abrasive receiving sheets such as bond paper or uncoated laser print paper.
There remains an ongoing need for fuser belts having durable surface layer compositions that adhere well to the substrate, form a hard, tough surface that is resistant to wear, cracking and solvents, and are capable of producing multiple high quality, high gloss toner images, including multicolor images. This need is well met by the release layer composition of the present invention.